Increasing demands in fuel efficiency have made hybrid systems more attractive in the automotive industry. In addition to a conventional combustion engine, an electric motor is an important part of the hybrid system. To reduce the manufacturing cost of the hybrid system, many of the electric motors used in the hybrid systems are induction motors with cast aluminum or copper squirrel cage rotors. An operating characteristic of these induction motors, known as “slip,” is usually proportional to the electric resistance in the rotor, particularly in the conductor bars. A lower resistant rotor produces lower slip and greater efficiency at load-carrying operating points. The electrical resistivities of pure copper and pure aluminum without defects (such as voids, cracks, or oxide inclusions, etc) are 17.1 nΩm and 26.5 nΩm, respectively. Consequently, for the same electric current requirement, using a copper rotor can lead to 35.5% reduction in resistance loss in comparison with aluminum rotor ((26.5−17.1)126.5=35.5%).
Because of its high density and melting point, however, copper has limitations and/or unique problems in rotor applications, particularly for hybrid systems. In hybrid applications, a high speed (e.g., more than 10,000 rpm) electric motor is usually needed due to space limitations in automotive vehicles. High density copper can produce very high centrifugal force and inertia at high rpm, and can significantly reduce the durability of the electric motor. Premature damage in rotor bearings is often seen in practice. In addition, copper rotors are usually manufactured by high pressure die casting (HPDC). The high melting point of copper (1083° C.) significantly reduces die life and increases the manufacturing cost of copper rotors.
Although cast aluminum rotors (bars and end rings together) overcome the shortfalls of high rotating inertia and low die life associated with copper material, the low mechanical properties, in particular, the low electric (pure Al: IACS 62%) and thermal conductivity of aluminum alloys, particularly when the cast aluminum conductor bars have casting defects including hot-tear cracks, porosity, and oxide inclusion etc, impose a great challenge for their successful application in electric motors. In addition, the aluminum alloys used to cast rotor squirrel cages are usually high purity aluminum, high purity aluminum casting alloys, or electric grade wrought alloys which are all difficult to cast because of their low fluidity, high shrinkage rate (density change from liquid to solid), high melting temperature, and large freezing range (temperature difference between liquidus and solidus), etc. These characteristics of the higher purity aluminum alloys increase porosity and the tendency of hot tearing, particularly at the locations where the conductor bars connect to the end rings, which leads to fracture between the conductor bars and the end rings. Furthermore, many cast aluminum squirrel rotor cages are made by high pressure die casting process in order to fill the thin and long bars (squirrel slots) in the laminate steel stack quickly to avoid cold shuts. The entrained air and abundant aluminum oxides produced during the high pressure die casting process, which are due to very high flow velocity (about 60 m/s) in mold filling, can not only decrease rotor quality and durability, but also significantly reduce the thermal and electric conductivity of the rotor, particularly in the conductor bars. In practice, it is often seen that the electric conductivity of the cast aluminum rotor (casting conductor bars and casting end rings) is only about 40-45% IACS. Because of the casting defects present in the cast aluminum conductor bars, the bars may be broken during motor operation. The broken bars will further reduce rotor conductivity and motor performance.
Therefore, there is a need for an improved rotor for an electric machine, and for methods of making improved rotors.